1. Field of the Invention
The present invention relates to a magnetic bearing device for use in, for example, a turbine unit employed in an air cycle refrigerating system and, in particular, to a motor built-in magnetic bearing device, in which a rolling bearing unit and a magnetic bearing unit are used in combination with each other used, with the magnetic bearing unit supporting one or both of an axial load and a bearing preload.
2. Description of the Prior Art
The air cycle refrigerating system makes use of an air as a coolant and does therefore fail to exhibit a sufficient energy efficiency as compared with the refrigerating system using chlorofluorocarbon or ammonium, but is considered agreeable in terms of environmental protection. In a facility such as, for example, a cold storage warehouse into which a refrigerating air can be directly blown, the total cost can be lowered if a cooling fan device and/or defroster are dispensed with, and, accordingly, the use of the air cycle refrigerating system in such application has been suggested. (See, for example, the Japanese Patent No. 2623202.)
Also, it is well known that in a deeply cold range of −30 to −60° C., the theoretical efficiency of air refrigeration is equal to or higher than that of chlorofluorocarbon or ammonium. It is, however, said that optimally designed peripheral equipments are needed in order to secure the theoretical efficiency of the air refrigeration. Those peripheral equipments include, for example, compressors and/or expansion turbines.
For the compressor and expansion turbine, a turbine unit, in which a compressor rotor and an expansion turbine rotor are mounted on a common main shaft, is generally utilized. (See the Japanese Patent No. 2623202.)
It is to be noted that for the turbine unit used to handle a process gas, a magnetic bearing type turbine unit has been suggested, in which the turbine rotor and the compressor rotor are respectively mounted on respective opposite ends of the main shaft, which is supported by a journal bearing and a thrust bearing that can be controlled by an electric current flowing through an electromagnet. (See the Japanese Laid-open Patent Publication No. 07-91760.)
Also, although related to a suggestion concerning a gas turbine engine, the use of a thrust magnetic bearing device has been made to reduce the thrust load, acting on the rolling bearing device for the support of a main shaft, in order to avoid the possibility that the thrust load would lead to reduction in bearing lifetime. (See, for example, the Japanese Laid-open Patent Publication No. 08-261237.)
As discussed above, in order to secure the theoretical efficiency of the air cooling, at which a high efficiency can be obtained in the deeply cold range, the air cycle refrigerating system requires the use of a compressor and an expansion turbine that are optimally designed.
For the compressor and the expansion turbine as mentioned above, the turbine unit including the compressor rotor and the expansion turbine rotor both mounted on a common main shaft is utilized. This turbine unit increases the efficiency of the air cycle refrigerator in view of the fact that the compressor rotor is driven by a power induced by the expansion turbine.
However, in order to secure a practically acceptable efficiency, a clearance delimited between each of the rotors and a housing must necessarily be small. Change in clearance constitutes a cause of an unstable operation during high speed rotation and, therefore, the efficiency tends to be lowered.
Also, by the effect of air acting on the compressor rotor and the turbine rotor, the thrust force acts on the main shaft and the bearing unit supporting the main shaft is loaded with the thrust load. The rotational speed of the main shaft in the turbine unit employed in the air cycle refrigerating system is 80,000 to 100,000 revolutions per minute, which is considerably high as compared with that in the bearing unit for the standard application. For this reason, the thrust load such as described above tends to bring about a reduction in long term durability and lifetime of the bearing unit used to support the main shaft and, in turn, a reduction in reliability of the air cycle refrigerating turbine unit. Unless the problem associated with the reduction in the long-term durability of the bearing unit is resolved, the air cycle refrigerating turbine unit can be hardly placed in practical use. However, the technology disclosed in the Japanese Patent No. 2623202 has not yet resolved the problem associated with the reduction in long-term durability of the bearing unit relative to the loading of the thrust load under such a high speed revolution.
In the case of the turbine compressor of a magnetic bearing type such as disclosed in the Japanese Laid-open Patent Publication No. 07-91760, in which the main shaft is supported by the journal bearing unit and the thrust bearing unit, both in the form of a magnetic bearing, the journal bearing unit lacks a function of regulating in the axial direction. For this reason, the presence of a factor or the like that render the control of the thrust bearing unit to be unstable makes it difficult to achieve a stabilized high speed revolution while the minute clearance is maintained between the rotor and the diffuser. The magnetic bearing unit involves a problem associated with a contact between the rotor and the diffuser at the time of failure of the electric power supply.
In view of the above, in order to alleviate the foregoing problems, the inventors of the present invention have developed a motor incorporated magnetic bearing device of such a structure as shown in FIG. 15. Specifically, the motor incorporated magnetic bearing device so developed includes, in a turbine unit for use in an air cycle refrigerating system, a compressor rotor 46a of a compressor 46 and a turbine rotor 47a of an expansion turbine 47, which are mounted on opposite ends of a main shaft 53, respectively; rolling bearing units 55 and 56 for supporting a radial load acting on the main shaft 53; an electromagnet 57 for supporting an axial load acting on the main shaft 53; and a motor 68 arranged coaxially of the main shaft 53 for providing a driving force that is cooperable with a driving force brought about by the turbine rotor 47a to drive the compressor rotor 46a. The electromagnet 57 used to support the axial load is arranged in face-to-face relation without contact to a thrust plate 53a, which is mounted coaxially on the main shaft 53 so as to extend radially outwardly therefrom, and is controlled by a magnetic bearing controller 59 for the magnetic bearing unit in response to an output generated from a sensor 58 for detecting a force acting in the axial direction. The motor 68 is of an axial gap type and includes a motor rotor 68a, formed in a thrust plate 53b which is different from the thrust plate 53a and mounted coaxially on the main shaft 53 so as to extend radially outwardly therefrom, and a motor stator 68b held in an axially face-to-face relation to the motor rotor 68a. The motor 68 referred to above is controlled by a motor controller 69 independently of the electromagnet 57. Also, the stator 68b of the motor 68 is of a core equipped structure having a coil 68bb wound around an axially extending stator yoke 68ba. 
In the motor incorporated magnetic bearing device of the structure described above, since a thrust force acting on the main shaft 53 can be supported by the electromagnet 57, the thrust force that acts on the rolling bearing units 55 and 56 can be relieved while suppressing an increase of the torque on a non-contact basis. As a result thereof, minute clearances between the rotors 46a and 47a and housings 46b and 47b, respectively, can be maintained at a constant value, allowing the long term durability of the rolling bearings relative to the load brought about by the thrust load.
It has, however, been found that the motor incorporated magnetic bearing device of the structure described above involves such a possibility that when the motor 68 operates in a high load region and an excessive axial load acts then, the control system for the magnetic bearing units 55 and 56 will become unstable. In other words, in the event that the excessive axial load acts, not only does the negative stiffness (which acts in a direction of displacement and a force thereof tends to increase as the displacement becomes large) in the electromagnet 57 increase, but also the negative stiffness of a magnetic coupling formed between the motor rotor 68a and the stator yoke 68ba, both forming respective parts of the motor 68, increase. For this reason, in the event that the negative stiffness of a composite spring formed by the electromagnet 57 and the motor 68 comes to be higher than the stiffness of the composite spring formed by the rolling bearing units 55 and 56 and respective support systems therefor, the control system for the magnetic bearing device will become unstable. In order to alleviate such a condition, the controller 59 is required to be provided with a phase compensating circuit or the like, resulting in complication in structure of the controller 59.